1. Field of the Invention
The present invention relates to a control unit, and more particularly to a control unit for an electronically controlled throttle valve which is capable of raising response speed of an electronically controlled throttle valve having a structure in which an acceleration pedal and the throttle valve are not mechanically connected to each other.
2. Description of the Related Art
Hitherto, control of the number of revolutions of an internal combustion engine mounted on a vehicle has been performed in accordance with an amount of depression of an acceleration pedal disposed in a driver's compartment adjacent to a foot of a driver. That is, internal combustion engines have generally incorporated a throttle valve disposed in a suction passage of the engine with the throttle valve connected to the acceleration pedal by a wire. When the acceleration pedal is depressed, the opening of the throttle valve is enlarged. Thus, an amount of air admitted into the internal combustion engine is enlarged, leading to an increased consumption of fuel. As a result, the number of revolutions of the internal combustion engine is enlarged.
Recent advances with computers have lead to the increased use of electronically controlled internal combustion engines which optimally control revolution speed of the engine. Electronic control of the internal combustion engine, for example, control of an amount of fuel injection, control of an ignition timing, and control of a timing at which a suction/exhaust valve operates have been previously developed, and have been followed by the practical application of the electronic control of the throttle valve.
The structure of an electronically controlled throttle valve unit is shown in FIG. 1. The electronically controlled throttle valve unit 20 incorporates a throttle lever 16 connected to an acceleration pedal (not shown) by a wire; an accelerator opening sensor 15 contained in the throttle lever 16 for detecting an opening of an accelerator corresponding to an amount of depression of the accelerator pedal; an engine control unit (hereinafter called an "ECU") 10 to which the opening of the accelerator detected by the accelerator opening sensor 15 is input; a throttle motor 4 for opening/closing a throttle valve 3 disposed in a suction passage 2 of the internal combustion engine in accordance with an output of the ECU 10; a throttle opening sensor 5 for detecting an opening of the throttle valve 3; a lever 17 for withdrawal running; a return spring 18 for the throttle valve 3; and a relief spring 19 for the lever 17 for withdrawal running. The throttle motor 4 has a built-in electromagnetic clutch.
In the electronically controlled throttle valve unit 20 structured as described above, when the acceleration pedal is depressed in accordance with the intention of a driver, the amount of depression of the acceleration pedal is transmitted to the throttle lever 16 by the wire. As a result, the throttle lever 16 is rotated. The throttle lever 16 includes the accelerator opening sensor 15. In accordance with the angle of rotation of the throttle lever 16, the amount of depression of the acceleration pedal is detected. The amount of depression of the acceleration pedal detected by the accelerator opening sensor 15 is sent to the ECU 10. The ECU 10 determines the opening of the throttle valve 3 in accordance with the detected amount of depression of the acceleration pedal so as to rotate the throttle motor 4. The opening of the throttle valve 3 is detected by the throttle opening sensor 5 so as to be fed back to the ECU 10. The throttle motor 4 must be a motor exhibiting quick response and small power consumption.
To perform the above-mentioned control, a signal transmitted from the throttle opening sensor 5 for detecting the opening of the throttle valve 3 is used. Moreover, a feedback control of the throttle motor 4 is performed by using proportion (P), integration (I) and differentiation (D) (hereinafter simply called "PID control") to eliminate deviation from the signal transmitted from the accelerator opening sensor 15.
In recent years, electronic throttle apparatus have been suggested which are structured such that the wire between the acceleration pedal and the throttle valve 3 is eliminated. The foregoing electronic throttle apparatus incorporate a rotational-angle sensor provided for a support shaft of the acceleration pedal. As an alternative to this, a stroke sensor for the acceleration pedal is provided. The value detected by the sensor is directly input to the ECU 10.
The ECU 10 determines the opening of the throttle valve 3 in response to a signal representing an opening of the acceleration pedal. Thus, the ECU 10 directly outputs an operating signal to the throttle motor 4. The opening of the throttle valve 3 is detected by the throttle opening sensor 5 so as to be fed back to the ECU 10. Note that the throttle opening sensor 5 may be contained in the throttle motor 4.
The control constants of the PID control including terms P, I and D have been fixed values determined by a tuning operation to satisfy specifications required for all of the running states of the system. Therefore, the conventional control unit for the electronically controlled throttle valve using the PID control cannot provide an optimum value for each running state of the engine. As a result, response and stability of the throttle valve 3 deteriorate.
To improve response of the operation of the throttle valve with respect to the acceleration pedal, an attempt has been made to enlarge the gain in the PID control. The foregoing structure encounters another problem of causing overshoot at the time of acceleration and undershoot at the time of deceleration. To improve response of the operation of the throttle valve with respect to the acceleration pedal, a structure has been employed in which sampling cycles for detecting the opening of the throttle valve are shortened to quickly follow a target value (commanded value) in the PID control. If the sampling cycles are shortened to reduce the controlling intervals of the throttle motor 4, overshoot and undershoot may easily occur.
Therefore, Japanese Patent Application Laid-Open No. HEI 8-326561 has been disclosed to overcome the problem of the overshoot and undershoot with respect to the target value of the opening of the throttle valve. According to the foregoing disclosure, a method has been suggested with which the PID control of the throttle valve is performed such that a state of the operation of the throttle valve is determined. If the determination is made that the throttle valve is being operated in a state in which the opening is larger than the target opening which is determined in accordance with the amount of depression of the acceleration pedal, it is determined that overshoot of the throttle valve has occurred. Thus, the gain (the differential term D) for use in the PID control is changed.
If the gain is changed after the determination of the overshoot of the throttle valve as is suggested in Japanese Patent Application Laid-Open No. HEI 8-326561, the throttle valve has already been within the overshoot region. Therefore, there arises a problem of insufficient response to restore the throttle valve to a normal operation state.